The overall goal of this work is to provide a detailed understanding of the effect of anterior cruciate ligament injury on knee movement in those who compensate well for the injury and those who do not. Some persons (copers) are able to fully compensate for the absence of the anterior cruciate ligament (ACL) while others (non-copers) are not. Non-copers demonstrate quadriceps femoris weakness, and use kinematic, kinetic, and muscle activity patterns that stiffen the knee joint for stability. They accomplish the joint stiffening via general cocontraction of the muscles around the knee and by reducing the force with which the foot hits the ground. Copers have no quadriceps weakness, normal ground reaction forces, and possess an ability to coordinate the activity of the lower extremity muscles to efficiently distribute control of the knee among the hip, knee and ankle while maintaining normal knee motion. Even using sophisticated motion analysis techniques, copers are indistinguishable from uninjured subjects. A new approach to in vivo analysis of musculoskeletal dynamics uses Cine-phase contrast (Cine-PC) magnetic resonance imaging (MRI) to image and track the moving knee. Cine-PC MRI, a non-invasive technique, is capable of measuring 3D muscle fiber and skeletal velocity, in vivo, during dynamic tasks. Through integration, 3D musculoskeletal movement can be tracked. A combination of the use of this new technology and conventional MRI, electromyography, and musculoskeletal modeling will provide a unique opportunity to elucidate the compensation strategies employed by the copers. There are two aims to this proposal. Aim I is to identify differences in knee kinematics, ligament lengths, tendon lengths, and muscle activation patterns of ACL deficient patients using Cine-phase contrast MRI and electromyographic analysis that characterize the mechanisms with which the copers, in altering their muscle activation pattern, alter their knee joint kinematics in order to stabilize their knees. Aim II is to identify differences in muscle activation patterns in ACL deficient copers and non-copers using electromyography and biomechanical modeling. Patient specific models of the ACL deficient knee using T1-weighted MRI will be developed and used as input to a biomechanical analysis. Previous work suggests that patients with ACL deficiencies balance knee joint loads between muscles and ligaments using a strategy that is different than that employed by unimpaired subjects. This will be examined for copers and non-copers in this study.